This invention relates to aqueous low temperature curable surface coating compositions. More particularly, it relates to a process for preparing improved water dispersed curable coating compositions comprising an aminoplast, a hydroxycopolymer and a hydroxyl containing resin.
It is well known in the coatings art to prepare coating compositions by dissolving in organic solvents an alkoxymethyltriazine or alkoxymethylurea and a hydroxy functional resin to provide coatings which exhibit satisfactory hardness. Because of greater environmental concerns today, efforts are being made to convert from solvent-based coatings to water-based coatings. Efforts are also being made to cure the coatings at low temperature to conserve energy and to utilize such coatings on substrates such as wood, paper, paper board and plastics that cannot survive high temperature cure. While it has been possible to use methoxymethylmelamines with hydroxyl functional resins in water based coatings to get satisfactory coating performance when the coatings are cured at high temperature, low temperature cure gives coatings that are soft, have poor water resistance and low gloss.
In solvent coating systems the addition of a styrene-allyl alcohol copolymer has been found to improve the properties of coatings cured at low temperatures. Until now, however, styrene-allyl alcohol copolymers have not been used with aqueous coating systems because of the insolubility of the copolymers and the difficulty of dispersing them in aqueous media.
In one attempt to use styrene-allyl alcohol copolymers, a solution of styrene-allyl alcohol copolymer in a methoxymethylmelamine was prepared and the solution was added to an aqueous dispersion of a hydroxy functional resin. The solution was prepared by adding the styrene-allyl alcohol copolymer to the aminoplast resin to give a concentration of up to 10 wt. % of the styrene-allyl alcohol copolymer by heating the aminoplast to the melting point of the styrene-allyl alcohol copolymer, about 100.degree. C. or higher and sprinkling in the styrene-allyl alcohol copolymer as a powder. However, such solutions give viscosities of greater than 10,000 centipoise apparently because of interaction between the allyl alcohol groups and the aminoplast and are not dispersible in aqueous dispersions of hydroxy-functional resin. At temperatures below about 80.degree. C., where interaction is minimized, styrene-allyl alcohol copolymer cannot be dissolved in the aminoplast resin even after lengthy periods of agitation. If the styrene-allyl alcohol copolymer is first melted and super-cooled to about 80.degree. C. or lower, and added to the aminoplast, it immediately precipitates out and does not form the desired solution.
We have now discovered a method for dispersing a styrene-allyl alcohol copolymer in aqueous dispersions of hydroxy functional resins containing an aminoplast.
Our method is achieved by
(1) forming a concentrated solution of an aminoplast and a styrene-(meth)allyl alcohol in a polar solvent and
(2) adding to the solution an aqueous solution or dispersion of one or more hydroxy functional resins and mixing to obtain a uniform dispersion.
Another aspect of our invention is directed to the coating compositions provided by our process and to substrates coated with such compositions.